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January 11, 2005

Climate Change Directly Impacts Earth’s Shape

by Sam Savage

Goddard Space Flight Center -- Scientists using NASA satellite data found the shape of the Earth appears to be influenced by big climate events that cause changes in the mass of water stored in oceans, continents and atmosphere.

The study's principal researchers are Minkang Cheng and Byron D. Tapley, of the Center for Space Research, University of Texas at Austin. They reviewed climate events like El NiÃ´o Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) that affect the amount of water moving in the oceans, atmosphere and continents.

The study shows significant variations in the shape of the Earth, defined by the Earth's gravity field, or geoid, during the past 28 years might be partially linked to climate events. The study examined Earth's oblateness, how much its rounded shape flattens at the poles and widens at the equator. Scientists measured the distance from ground stations to satellites by using Satellite Laser Ranging (SLR) data that are accurate within one millimeter.

The data reflected mass changes as water redistributed in oceans, atmosphere, and in soil. The redistribution resulted in slight changes of the Earth's gravity field, detectable with geodetic satellites, those that study of the size and shape of the Earth.

The researchers found over the past 28 years, two large variations in the Earth's oblateness were connected to strong ENSO events. Variations in mass distribution, which caused the change in the gravity field, were predominantly over the continents, with a smaller contribution due to changes over the ocean. The cause of a variation in the Earth's mass over the 21-year period between 1978 and 2001, however, still remains a mystery.

The scientists also found that another change in mass distribution may have started in late 2002, which coincides with the moderate El NiÃ´o that developed at that time.

"The main idea, however, is that the Earth's large scale transport of mass is related to the long-term global climate changes," said Cheng.

Cheng and Tapley's research relied on NASA's SLR data to measure changes in the longest wavelengths of the Earth's gravity field in order to see how the global-scale mass was redistributed around the world.

The Earth's gravity is an invisible force of attraction that pulls masses together. The relative motion of a small lighter object, such as a spacecraft, to a large heavy object such as the Earth, depends on how much mass each object has and how that mass is distributed.

Scientists can measure the changes in Earth's gravitational pull using instruments on the ground to track satellites in space. So, water mass shifts on Earth and the changes in shape of the Earth can be detected.

The long-term history of the SLR measurements make it possible for scientists to see the changes over time in melting glaciers and polar ice sheets and the associated sea level change. The SLR data have also been used to detect the motion of global tectonic plates on which landmasses rest, the deformation of the Earth's crusts near plate boundaries, and the orientation and rate of spin of the Earth.

In March 2002, NASA and the German Aerospace Center launched the Gravity Recovery and Climate Experiment (GRACE) to sense small-scale variations in Earth's gravitational pull from local changes in Earth's mass.

GRACE data will assist with future studies similar to Cheng and Tapley's research. The GRACE satellite, together with 18 other NASA research satellites, have opened new windows to exploring Earth and to understanding the intricate processes that support life.

The study was published in a recent issue of the Journal of Geophysical Research-Solid Earth.